1. Field of the Invention
The present invention relates to an optical film useful as a protective film for a polarizing plate and the like, and to a polarizing plate and a liquid-crystal display device employing it.
2. Related Art
Optical compensation sheets are used in various liquid-crystal display devices for solving a problem of panel coloration and for widening a viewing angle. Proposed is use of an optical compensation sheet having, as formed on a transparent support, an optically-anisotropic layer comprising liquid crystal molecules. It is known to dispose an alignment layer between the transparent support and the optically-anisotropic layer of the optical compensation sheet (for example, see JPA Nos. hei 7-191217, hei 8-21996 and hei 8-94838). The optical compensation sheet of the type requires adhesiveness between the transparent support and the alignment layer, and adhesiveness between the alignment and the optically-anisotropic layer comprising liquid crystal molecules. An alignment layer may be subjected to a treatment of rubbing, electric field application, magnetic field application, irradiation with light or the like to exhibit an aligning ability; however, fine dust or contaminants, if any, adhering to the alignment layer may detract from the uniformity in aligning ability. In particular, in rubbing treatment, the film surface is rubbed, and therefore the treatment requires a countermeasure to static charge generation. Accordingly, in general, a water-soluble resin cured film is used for the alignment layer, and in particular, a hydroxyl group-having resin such as polyvinyl alcohol based is used for it.
On the other hand, a preferable protective film for a polarizing plate is optically transparent, and shows small birefringence and a smooth surface; and therefore, a cellulose triacetate (hereinafter this may be referred to as TAC) film is mainly used for it. A liquid crystal display employing a polarizing plate having a cellulose triacetate film as its protective film sometimes suffers from display image unevenness. This may be due to dimensional change of the polarizing film depending on the ambient temperature and humidity fluctuation in long-term use. More specifically, the unevenness may be caused by the following reasons. When a polarizer film, formed of a PVA film, is exposed to various temperature/humidity fluctuation conditions, the PVA film may undergo significant dimensional change owing to moisture penetration and evaporation in and from it. The environment-dependent dimensional change may conduct from the PVA film to other members such as optical compensation sheet, adhesive and liquid-crystal cell, as a deformation stress, and the members may undergo change of their optical properties, which may cause light leakage. Accordingly, it is expected to reduce moisture permeability of a protective film of a polarizing plate so as to inhibit moisture penetration and evaporation in and from it, thereby improving the durability of the polarizing plate and preventing display unevenness to be caused by light leakage.
For sticking cellulose triacetate, which is to be a protective film or a retardation sheet, to a polarizing film, known is a method of hydrophylicating the surface of cellulose triacetate to be stuck to a polarizing film by alkali saponification treatment (for example, see JPA No. hei 7-151914 (e.g., paragraph [0008]), JPA No. hei 8-94838 (e.g., paragraph [0033]), JPA No. 2001-166146 (e.g., paragraph [0083]), and JPA No. 2001-188130 (e.g., paragraph [0042]).
The vinyl alcohol-based resin layer used as an alignment layer of retardation sheet is soluble in water, and therefore, its alkali saponification lowers the bonding force between the inside of the vinyl alcohol-based resin layer and the interface of the layer whereby the layer may peel off or, owing to long-term or rapid environmental change, especially temperature change, the durability of polarizing plate may worsen.
A liquid-crystal display device is usually left in use for a long period of time; and, therefore, the polarizing plate used in the device is required to have long-term durability with no trouble of appearance change such as peeling and no trouble of polarizing property deterioration even in long-term use under varying temperature/humidity conditions.